One of the most exciting prospects for optical fibres made from fluoride glasses is the possibility of providing long distance optical communication systems without the need for repeaters. This objective has stimulated much of the work into fluoride glasses over the past ten years, and has prompted the writing of this book. It has also emerged that fluoride fibres can transmit both visible and infrared energy (from about 0.5 to 5 ,urn) and that they have many applications outside the field of telecommunications. These include optical fibre sensors (particularly in remote infrared spectroscopy), laser surgery and fibre lasers. Several companies are now established in the field, and good quality fluoride fibres are available from sources throughout the USA, Europe and Japan. Moreover, the first commercial instruments based on fluoride fibres are finding their way to the market place and these fibres will undoubtedly form the basis of many more instruments yet to be developed. The work presented in this book represents the field both from an academic understanding of the materials and ways to convert them into fibre, and from a practical and commercial viewpoint. The principal author and some of the co­ authors are based at the British Telecom Research Laboratories in the UK.

Inhalt

1 Perspective and overview.- 1.1 Introduction.- 1.2 Optical fibre fundamentals.- 1.3 Loss mechanisms in optical fibre materials.- 1.3.1 Power losses and units.- 1.3.2 Extrinsic and intrinsic loss mechanisms.- 1.3.3 Intrinsic transparency: mechanisms and models.- 1.4 Long wavelength fibre materials.- 1.4.1 Materials, choices and sources of information.- 1.4.2 Silica-based fibres.- 1.4.3 Crystalline fibre materials.- 1.4.4 Chalcogenide glasses.- 1.4.5 Hollow infrared waveguides.- 1.5 Fluoride glasses and optical fibres.- 1.5.1 Concepts, definitions and materials.- 1.5.2 Fluoride fibre technology: a brief personal history.- 1.6 Applications for infrared optical fibres.- 1.7 Future prospects.- References.- 2 Properties of fluoride glasses.- 2.1 Introduction.- 2.2 Glass-forming systems, structure and crystallization.- 2.2.1 Structural models.- 2.2.2 Crystallization behaviour.- 2.3 Thermal properties.- 2.3.1 Viscosity.- 2.3.2 Thermal expansion behaviour.- 2.3.3 Diffusion.- 2.3.4 Thermal conductivity.- 2.3.5 Heat transfer.- 2.4 Other properties.- 2.4.1 Density.- 2.4.2 Gas solubility.- 2.4.3 Elastic moduli.- 2.4.4 Microhardness.- 2.5 Optical properties.- 2.5.1 Infrared absorption.- 2.5.2 Ultraviolet absorption.- 2.5.3 Intrinsic scattering loss.- 2.5.4 Minimum intrinsic losses.- 2.5.5 Refractive index and dispersion.- 2.5.6 Fluorescence.- References.- 3 Propagation in optical fibres.- 3.1 Introduction.- 3.2 Multimode fibres.- 3.2.1 Propagation in multimode fibres.- 3.2.2 Attenuation.- 3.2.3 Real fibres and fibre links.- 3.3 Monomode fibres.- 3.3.1 Propagation in monomode fibres.- 3.3.2 Optimized fibre design.- 3.3.3 Non-linear effects.- References.- 4 Manufacture of infrared fibres.- 4.1 Materials preparation.- 4.1.1 Introduction.- 4.1.2 Synthetic routes.- 4.1.3 Purification.- 4.2 Melting techniques.- 4.2.1 Melting environments.- 4.2.2 Containment vessels.- 4.2.3 Reactive atmosphere processing.- 4.2.4 Homogenization and fining.- 4.2.5 Crystallization.- 4.3 Fibre fabrication.- 4.3.1 Glass compositions.- 4.3.2 Glass melting.- 4.3.3 Preform fabrication.- 4.3.4 Fibre drawing.- 4.3.5 Other techniques.- 4.3.6 Monomode fibre.- 4.4 Problems.- 4.5 Fibre results.- References.- 5 Intrinsic loss measurements.- 5.1 Introduction.- 5.2 Rayleigh scattering.- 5.3 IR multiphonon edge.- 5.4 Minimum intrinsic loss.- 5.5 Longer wavelength transmitting fibres.- References.- 6 Extrinsic absorption.- 6.1 Introduction.- 6.2 Spectrometer measurements.- 6.2.1 Nomenclature.- 6.2.2 Experimental.- 6.3 Absorption due to water.- 6.3.1 OH- in oxide glasses.- 6.3.2 OH- in fluoride glasses.- 6.3.3 OH- in ZrF4 IR fibres.- 6.4 Absorption due to transition metal ions.- 6.4.1 Introduction.- 6.4.2 Oxidation-reduction equilibrium.- 6.4.3 Ligand field theory.- 6.4.4 Absorption spectra.- 6.4.5 Discussion.- 6.5 Absorption due to rare earth ions.- 6.5.1 Introduction.- 6.5.2 Oxidation states.- 6.5.3 Electronic spectra.- 6.5.4 Absorption spectra.- 6.5.5 Discussion.- 6.6 Other impurities.- 6.6.1 Dissolved gases.- 6.6.2 Molecular ions.- 6.6.3 Reduced species.- 6.6.4 Oxide absorption.- References.- 7 Extrinsic scattering.- 7.1 Introduction.- 7.2 Identification of scattering centres.- 7.2.1 Large crystals.- 7.2.2 Gas bubbles.- 7.2.3 Sub-micron centres.- 7.2.4 Deviations of fibre geometry.- 7.3 Scattering theory.- 7.4 Scattering solutions for limiting cases.- 7.4.1 Small spheres, intermediate index (Rayleigh scattering).- 7.4.2 Small high index spheres.- 7.4.3 Intermediate size spheres with m close to 1 (Rayleigh-Gans).- 7.4.4 Intermediate size sphere with higher refractive index.- 7.4.5 High index (conducting) spheres of small size.- 7.4.6 High index spheres of intermediate size.- 7.4.7 Large spheres with index close to 1.- 7.4.8 Large spheres with high refractive index.- 7.5 Solutions for metallic (absorbing) spheres.- 7.6 Applications to practical examples.- 7.7 Calculations of scattering loss.- 7.8 Scattering loss in fibres.- 7.9 Conclusions.- References.- 8 Measured losses in fibres.- 8.1 Introduction.- 8.2 Available techniques.- 6.2.1 Sources and detectors.- 8.2.2 Total loss.- 8.2.3 Absorption.- 8.2.4 Scattering.- 8.3 Total loss.- 8.4 Absorption.- 8.5 Scattering.- 8.6 Recent results.- 8.7 Predicted minimum loss.- References.- 9 Mechanical properties.- 9.1 Introduction.- 9.2 Fracture and glass parameters.- 9.3 Strength measuring methods.- 9.3.1 What is strength?.- 9.3.2 Tensile measurements.- 9.3.3 Bend measurements.- 9.4 Fibre strengths.- 9.4.1 Mechanical polishing.- 9.4.2 Chemical polishing.- 9.5 Identification of flaws and effect on strength.- 9.6 Ultimate strength and maximum practical strength.- 9.7 Strength and homogeneous crystal growth.- 9.8 Durability.- 9.8.1 Zero stress aging.- 9.8.2 Dynamic fatigue.- 9.8.3 Hermetic coatings.- 9.9 Conclusions.- References.- 10 Applications.- 10.1 Mid-IR communication systems.- 10.1.1 System demonstrations.- 10.2 Active fibres.- 10.2.1 Fibre devices.- 10.2.2 Absorption and fluorescence spectra.- 10.2.3 Fluoride fibre lasers.- 10.2.4 Upconversion.- 10.3 Laser surgery.- 10.4 Infrared fibre sensors.- 10.4.1 IR fibre chemical sensors.- 10.4.2 IR fibre non-chemical sensors.- References.